A triple quadrupole mass spectrometer (TOMS) has been designed and developed for the direct analysis of mixtures and the elucidation of molecular structures. In the TOMS, several ionic species are generated from a sample, ions of a particular mass are selected for fragmentation, and the resulting fragment ions are mass analyzed. The instrument consists of, in series, a dual chemical ionization/electron impact (CI/EI) ionization source, a quadrupole mass filter, an RF-Only quadrupole that can be pressurized with a collision gas, a second quadrupole mass filter, and an electron multiplier. The ion fragmentation process is performed by collision-induced dissociation (CID) in the RF-only quadrupole collision chamber. The instrument is described and its performance evaluated. For structure elucidation, the formation and fragmentation of every fragment ion from a compound can be directly determined. For mixture analysis, the first mass analyzer can separate the mixture components as their molecular ions, produced by soft ionization (e.g., CI) of the sample. The fragmentation spectra of the individual components can then be obtained by fragmenting the selected molecular ions and scanning the second analyzer. The identification of isomeric and isobaric compounds in a mixture is demonstrated. Detection limits of ten to the minus 15th power mole for methane and nitrobenzene are shown.